1. Field of the Invention
The present invention relates to the use of gas flushing in lithographic apparatus. More particularly, the invention relates to the use of such a gas flushing system in lithographic projection apparatus comprising:
a radiation system for supplying a projection beam of radiation;
a first object table for holding a mask;
a second object table for holding a substrate; and
a projection system for imaging an irradiated portion of the mask onto a target portion of the substrate.
2. Description of the Related Art
For the sake of simplicity, the projection system may hereinafter be referred to as the xe2x80x9clensxe2x80x9d; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, catadioptric systems, and charged particle optics, for example. The illumination system may also include elements operating according to any of these principles for directing, shaping or controlling the projection beam of radiation, and such elements may also be referred to below, collectively or singularly, as a xe2x80x9clensxe2x80x9d. In addition, the first and second object tables may be referred to as the xe2x80x9cmask tablexe2x80x9d and the xe2x80x9csubstrate tablexe2x80x9d, respectively. Further, the lithographic apparatus may be of a type having two or more mask tables and/or two or more substrate tables. In such xe2x80x9cmultiple stagexe2x80x9d devices the additional tables may be used in parallel, or preparatory steps may be carried out on one or more stages while one or more other stages are being used for exposures. Twin stage lithographic apparatus are described in International Patent Applications WO 98/28665 and WO 98/40791, for example.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the mask (reticle) may contain a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (which may comprise one or more dies) on a substrate (silicon wafer) which has been coated with a layer of photosensitive material (resist). In general, a single wafer will contain a whole network of adjacent dies that are successively irradiated via the reticle, one at a time. In one type of lithographic projection apparatus, each die is irradiated by exposing the entire reticle pattern onto the die at once; such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatusxe2x80x94which is commonly referred to as a step-and-scan apparatusxe2x80x94each die is irradiated by progressively scanning the reticle pattern under the projection beam in a given reference direction (the xe2x80x9cscanningxe2x80x9d direction) while synchronously scanning the wafer table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally  less than 1), the speed V at which the wafer table is scanned will be a factor M times that at which the reticle table is scanned. More information with regard to lithographic devices as here described can be gleaned from International Patent Application WO 97/33205.
To reduce the size of features that can be imaged, it is desirable to reduce the wavelength of the illumination wavelength. To such end, it has been proposed to use wavelengths of less than about 200 nm, for example 157 nm or 126 nm. However, such wavelengths are strongly absorbed by normal atmospheric air leading to unacceptable loss of intensity as the beam traverses the apparatus. To enclose the entire apparatus and operate in vacuum would introduce unacceptable delays in wafer and reticle exchange whereas to flush the entire apparatus with a gas which does not absorb the illumination wavelength, such as ultra-pure nitrogen (N2), would result in excessive operating costs due to the consumption of the gas in an imperfectly closed machine.
An object of the present invention is to provide a system for reducing absorption of the illumination and projection beams in a lithographic projection apparatus whilst avoiding detrimental effects on the throughput and maintenance overhead of the apparatus as well as reducing the use of expensive consumables.
According to the present invention there is provided a lithographic projection apparatus comprising:
a radiation system for supplying a projection beam of radiation;
a first object table for holding a mask;
a second object table for holding a substrate; and
a projection system for imaging an irradiated portion of the mask onto a target portion of the substrate; characterized by:
flushing gas means for generating a substantially laminar flow of flushing gas across at least a part of the path of said projection beam to displace ambient air therefrom, said flushing gas being substantially non-absorbent of said radiation of said projection system.
In embodiments of the present invention, the spaces traversed by the projection beam are flushed with a laminar flow of ultra-pure nitrogen (N2), or other gas (e.g. Helium, Argon or Xenon) transparent to the illumination radiation used. To ensure laminar flow and minimize turbulence, the various spaces are separated from one another and all parts are smoothed as far as possible. The effective Reynolds number of the system is thereby reduced, because of the reduction of the hydraulic diameter of the system and because relatively rough areas are covered. The flow speed of the nitrogen in each space is maintained higher than the maximum speed of any moving parts in that space and, in all cases, higher than the diffusion speed of air. In this way, and due to the minimization of turbulence vortices, contamination of the flushing gas is minimized and the gas may be recovered and re-used. Re-use of the gas may be in the same area from which it was recovered or may be elsewhere, e.g in a cascade fashion. In such an arrangement, fresh flushing gas is supplied to the most critical area(s) and then re-used in successively less critical areas. The flushing gas may of course be cleaned or scrubbed before re-use and mixed with fresh gas as desired to control contamination levels.
In this way, the invention minimizes absorption of the illumination radiation and minimizes consumption of flushing gas while avoiding the need for sealing the apparatus and hence minimizing downtime in wafer and reticle exchange.
According to a further aspect of the invention there is provided a method of manufacturing a device using a lithographic projection apparatus including:
a radiation system for generating an illumination beam;
a first object table for holding a mask;
a second object table for holding a substrate; and
a projection system for imaging irradiated portions of the mask onto targetportions of the substrate; the method comprising the steps of:
providing a mask bearing a pattern to said first object table;
providing a substrate provided with a radiation-sensitive layer to said second object table;
irradiating portions of the mask and imaging said irradiated portions of the mask onto said target portions of said substrate; characterized by the step of:
providing flushing gas to flow in a substantially laminar flow across at least a part of the beam path of said projection beam to displace therefrom ambient air, said flushing gas being substantially non-absorbent of said radiation of said projection system.
In a manufacturing process using a lithographic projection apparatus according to the invention a pattern in a mask is imaged onto a substrate which is at least partially covered by a layer of energy-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book xe2x80x9cMicrochip Fabrication: A Practical Guide to Semiconductor Processingxe2x80x9d, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4.
Although specific reference may be made in this text to the use of the apparatus according to the invention in the manufacture of ICs, it should be explicitly understood that such an apparatus has many other possible applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms xe2x80x9creticlexe2x80x9d, xe2x80x9cwaferxe2x80x9d or xe2x80x9cdiexe2x80x9d in this text should be considered as being replaced by the more general terms xe2x80x9cmaskxe2x80x9d, xe2x80x9csubstratexe2x80x9d and xe2x80x9ctarget portionxe2x80x9d, respectively.
In the present document, the terms illumination radiation and illumination beam should not be read as being restricted to the cited examples of 157 or 126 nm electromagnetic radiation; it is conceivable that other radiation wavelengths or types may be used in the present invention.